Whisking milk should be understood as being a process of frothing milk by mixing the milk with air without a use of steam. Although such a process is known, and devices have been developed for performing such a process, most devices which are suitable to be used for emitting milk froth and which are currently available in the market are adapted to use steam. In such devices, the steam is used for pumping the milk out of a container such as a cartridge or jug, heating the milk, pumping air, and pumping the obtained milk froth to an outlet of the device. However, using steam for making milk froth has disadvantages, as will be explained in the following.
A first disadvantage is that when steam is used, the milk is always heated. Therefore, in a situation in which it is desired to have cold milk froth, the froth needs to be cooled down after the frothing process has taken place. A second disadvantage is that in general, a steam supply is relatively difficult to control. A third disadvantage is that devices for generating steam need to be decalcified from time to time. A fourth disadvantage is that the use of steam is dangerous on the basis of the fact that it is a hot fluid which may cause injuries to a user. A fifth disadvantage is that the use of steam involves a construction which is relatively difficult to clean. It is a well-known fact that cleaning of a device for handling milk is very important, as milk residues may grow bad and spoil milk froth to be made in future applications of the device.
Using whisking overcomes the problems associated with using steam. However, known devices which are suitable for whisking milk require a lot of cleaning effort.
A device for whisking milk is known from US 2005/0233043. In particular, the device comprises a suction line and a pump, which are suitable to be used for suctioning cold milk from a container, and a continuous-flow heater for heating the milk, wherein the suction line is connected to an output line for delivering milk foam or a warm milk drink to another container such as a cup. Furthermore, the known device comprises an air supply line which opens into the suction line, and which serves for supplying air when it is desired to have milk foam.
After a process of producing milk foam or a hot milk drink, for the purpose of cleaning of the continuous-flow heater and the lines leading to and from the heater, a cleaning fluid can be introduced from a source via a valve and a line into the suction line, and pumped into the outlet line by means of the pump. US 2005/0233043 discloses that it is preferred if a certain quantity of water is pumped through the suction line after each flow of milk, so that a settling of milk residue can be prevented, especially in the continuous-flow heater. However, in the known device, a portion of the suction line, which is a portion extending between an end situated at the milk container and a non-return valve for preventing a back flow of milk, is not cleaned, as the line for supplying the cleaning fluid opens into the suction line at a position upstream of the non-return valve. Hence, it is possible for milk residue to settle in the said portion of the suction line, which is a disadvantage of the known device.